In a multiphase converter constituted by a plurality of conversion units capable of performing voltage conversion, the number of conversion units that perform voltage conversion is changed in accordance with a predetermined condition in order to efficiently perform voltage conversion. Patent Document 1 describes a power supply system that includes a plurality of power supplies connected in parallel and an operating number controller that controls the number of the power supplies in operation. The operating number controller determines the number of the power supplies in operation based on a load current in a case of operating a predetermined number of power supplies and the conversion efficiency relative to the load current. The technique described in Patent Document 1 is also described in Non-Patent Document 1. The technique described in Patent Document 3 is also described in Patent Documents 4 and 5. The technique described in Patent Document 6 is also described in Patent Documents 7 and 8.